Polymer grafted nanoparticles are very appealing composite materials ascribed to their broad applications in coatings, biomedical field and chemosensors. In addition, the properties of the composites can be tailored by choosing different substrate nanoparticles and polymer shells. The graft density and chain lengths of the surface attached polymers are able to greatly affect the dispersion and the final properties of the nanoparticles. Thus, controlling the growth of polymer on surfaces is extremely significant.
Poly(vinyl pyrrolidone) (PVP), an very important water soluble polymer, has been widely applied in biomedical area and cosmetic industry due to its nontoxic and nonionic characteristics, and its biocompatibility. It is particularly attractive in the drug delivery field ascribed to its abilities to conjugate active biomolecules and prolong the circulation lifetime of antibiotics in blood. Although PVP has been widely synthesized via free radical polymerization of N-vinyl pyrrolidone (NVP), the preparation of PVP grafted nanoparticles with great control on polymer chain length and architecture is desirable and extremely important for biomedical applications. Nitroxide Mediated Polymerization (NMP), atom transfer radical polymerization (ATRP) and organocobalt-mediated radical polymerization (OMRP) have demonstrated no effect or very little effect after careful design and operation, on the mediating polymerization of NVP in a controlled manner. The reason is the special polar lactam structure in the monomer, which interacts with the catalysts and further interrupts them in these polymerization systems.
Reversible addition-fragmentation chain transfer/macromolecular design by interchange of xanthates (RAFT/MADIX) polymerization has been widely used to synthesize polymers in a controlled manner (both molecular weight and polydispersity) since it does not require the use of inorganic catalysts and is adaptable to an extremely wide range of functional monomers. So far, O-ethyl xanthate based RAFT/MADIX agents have been used in mediating the NVP polymerization. However, it was reported that the terminal O-ethyl xanthate on PVP chains is unstable and decomposed in polymerization. In addition, the terminal O-ethyl xanthate on PVP chains is hydrolyzed to form a hydroxyl end group after 16 h in a 40° C. aqueous environment. Thus, more thermally stable RAFT/MADIX agents are desirable for mediating the polymerization of NVP.
Dithiocarbamates have been used as a thermally stable agent in RAFT polymerization. So far, only a few groups have reported using dithiocarbamates to mediate the polymerization of NVP. However, these dithiocarbamate RAFT agents contained either bulky Z groups or alkyne based primary R groups, which are not good leaving groups and interfere with the polymerization due to the alkyne moiety. Thus, new dithiocarbamate RAFT agents are needed to mediate the polymerization of NVP.
Only a few groups have reported placing PVP on nanoparticles. In these reported PVP covered particles, PVP chains are physically absorbed on the particles surfaces or grafted on surfaces without control (by free radical polymerizations). That is, PVP chains are physically absorbed on the particles surfaces, rather than growing covalently attached chains from particle surfaces in a controlled manner.
Thus, a need exists for mediating the polymerization of NVP on nanoparticles in a controlled manner via surface-initiated RAFT polymerization.